Many types of inhibitors are employed to enhance the shelf-stability of liquid thermosettable resins such as unsaturated polyester resins and vinyl ester resins. Examples of these inhibitors include hydroquinone and its alkylated derivatives, phenolic compounds with electron-withdrawing substituents such as nitro, nitroso, or halo moieties, and quinonoid compounds such as para-benzoquinone or para-chloranil.
These compounds enhance the shelf stability of the resin by preventing premature polymerization resulting in the resin becoming an unusable gelatinous mass during storage. However, the addition of such inhibitors to the resin formulation also reduces the ability of the resin to effectively cure during processing. Normally, the shelf stability of the resin must be balanced against the reactivity required for the intended application.
Some of this loss in reactivity may be overcome by the addition of accelerators such as N,N-dimethylaniline, N,N-dimethyl-para-toluidine, or N,N-dimethylacetoacetamide. These compounds accelerate the rate of initiator decomposition and thereby enhance the rate of curing. However, many of these accelerators are unstable if used as an additive in a pre-formulated resin, and may "poison" the cure of the resin by decomposing the initiator too quickly and providing too many radicals of low molecular weight which then forms an incompletely cured resin.
Synergists are compounds which interact together such that the combination of two or more additives mutually enhance the desired performance of each other. Such an example is the addition of phosphite esters in the presence of alkylphenols as antioxidants for polymers and drying oils. Another is the combination of oxalic acid dihydrate and phenothiazine to enhance the storage stability of vinyl ester resins.
Many types of quinonoid compounds have been known to form electron donor-acceptor complexes with compounds such as amines, alkylated polynuclear aromatic compounds, and many other types of polynuclear aromatic compounds. Quinonoid compounds have long been known to be most effective polymerization inhibitors; however, their use as shelf-stability enhancers have been severely limited by their drastic reduction of the desired polymerization rate and would require an impractical amount of initiator and accelerator to effectively complete the curing process. The addition of imidazole derivatives, which only act as weak accelerators in this process, serve to enhance both the shelf-stability and reactivity in the presence of many quinonoid compounds to a greater degree than would be found with inhibitors which do not form electron-donor acceptor complexes.
It would therefore be desirable to have available storage stable thermosettable ethylenically unsaturated resins which also have suitable reactivity rates with curing agents therefor.